Question

The electron affinity of oxygen is lower than that of sulphur. Explain.

Answer 1

1. Both oxygen and sulphur belong to group 16 and have high electron affinity due to their desire to gain electrons.
2. Oxygen has a smaller atomic size than sulphur, so its electrons are more tightly held.
3. In oxygen, the added electron enters a small 2p orbital, which already has paired electrons.
4. This causes strong electron-electron repulsion in oxygen, making it less eager to accept another electron.
5. Sulphur has larger 3p orbitals with lower electron density, so the added electron experiences less repulsion.
6. Hence, oxygen’s electron affinity is less negative (lower) than sulphur’s because it is harder to add an electron to oxygen’s compact 2p orbital.
7. Electron affinity decreases down the group due to increasing atomic size and decreasing effective nuclear charge, but this repulsion effect makes oxygen an exception.